The present invention generally relates to substrate processing method and more particularly to a nitriding method of an oxide film formed on a silicon substrate surface.
With progress in the art of device miniaturization, fabrication of ultrafine semiconductor devices having a gate length of less than 0.1 μm is now becoming possible.
In order to achieve improvement of operational speed of the semiconductor device with such ultrafine semiconductor devices by way of decrease of the gate length, there is a need to decrease the thickness of the gate insulation film according to scaling law. In the case of using a conventional thermal oxide film for the gate insulation film, for example, it is necessary to reduce the thickness of the gate insulation film to be equal to or smaller than the conventional thickness of 1.7 nm. However, such a decrease of thickness of the oxide film invites increase of the gate leakage current through the oxide film as a result of tunneling effect.
Thus, there have been studies to use a high-K dielectric film such as Ta2O5 or ZrO2 for the gate insulation film in place of the conventional silicon oxide film. However, these high-K dielectric films have a nature very much different from that of the silicon oxide film used conventionally in the semiconductor technology, and there remain numerous problems to be solved before such high-K dielectric film is used for the gate insulation film.
Contrary to this, a silicon nitride film has a material used conventionally in the semiconductor processes, and is thought as being a promising material for the gate insulation film of the next-generation high-speed semiconductor devices in view of its specific dielectric constant, which is twice as large as that of a silicon oxide film.
Conventionally, a silicon nitride film has been formed on an interlayer insulation film by a plasma CVD process. However, such a CVD nitride film generally has the feature of large leakage current, and the use thereof for a gate insulation film has been inappropriate. In fact, no attempts have been made conventionally to use a nitride film for a gate insulation film.
Meanwhile, there have been proposed recently the technology of nitriding a surface of a silicon oxide film and convert the same to an oxynitride film by generating N radicals or NH radicals by introducing a gas containing nitrogen such as a nitrogen gas, nitrogen and hydrogen gases or an NH3 gas into microwave-excited rare gas plasma of Ar, Kr, or the like. The oxynitride film thus formed has the feature of small oxide-film equivalent thickness and also the feature of leakage current characteristics comparable to or even surpassing that of a thermal oxide film, and thus, the oxynitride film thus formed is thought as being a promising material for the gate insulation film of the next-generation high-speed semiconductor devices. Further, the oxynitride film thus formed is chemically stable, and it is possible to suppress, in the case a high-K dielectric film is formed on the oxynitride film, the diffusion of metal elements in the high-K dielectric film through the oxynitride film and associated reaction of the high-K dielectric film with the silicon substrate caused by way of such diffusion. Further, there is proposed a technology of directly nitriding a silicon substrate surface by such microwave plasma.
Conventionally, it has been known to introduce nitrogen into an oxide film by a thermal annealing process conducted in nitrogen ambient or by an implantation of nitrogen ions. On the other hand, it is known that the nitrogen atoms introduced according to such a process predominantly concentrate in the vicinity of the interface between the silicon substrate and the oxide film. As a result, in the case such a conventional oxynitride film is used for the gate insulation film of a MOS transistor, there are caused problems such as variation of the threshold voltage or degradation of mobility caused by formation of the interface states.
Because of similar reasons, there can be caused deterioration of semiconductor device characteristics also in the case of an oxynitride film processed by N radicals or NH radicals is used, instead of the desired improvement of semiconductor device characteristics, unless the distribution of the nitrogen atoms in the film is controlled appropriately.